Polymer grade terephthalic acid (TA) is the starting material for polyethylene terephthalate (PET), which is the principal polymer for polyester fibers, polyester films, and resins for bottles and the like containers. Polyester fibers are used in textiles as well as in industrial applications such as tire cord. Polyester films coated with adhesives and emulsions are useful as wrapping tapes, photographic films, recording tapes, and the like.
Polymer grade terephthalic acid is derived from relatively less pure, technical grade terephthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst as described in U.S. Pat. No. 3,584,039 to Meyer. In the purification process, the impure terephthalic acid is dissolved in water at an elevated temperature, and the resulting solution hydrogenated, preferably in the presence of a hydrogenation catalyst, e.g., palladium on a carbon support, as described in U.S. Pat. No. 3,726,915 to Pohlmann. This hydrogenation step converts the various color bodies present in the relatively impure terephthalic acid to colorless products. The principal feedstock impurity, 4-carboxybenzaldehyde (4-CBA), is converted to p-toluic acid (TOL). Color-forming precursors and color bodies present as impurities are believed to be of the benzil, fluorenone and/or anthraquinone type.
The resulting purified product, polymer grade terephthalic acid, is recovered by crystallization, centrifugation, and drying. Another related purification-by-hydrogenation process for aromatic polycarboxylic acids produced by liquid phase catalytic oxidation of polyalkyl aromatic hydrocarbons is described in U.S. Pat. No. 4,405,809 to Stech et al.
At 4-CBA concentrations of about 100 parts per million, by weight, of TA, however, the rate of reduction of 4-CBA to TOL slows down considerably. The reduction rate may stop altogether at certain reaction conditions. Inasmuch as unreduced or partially reduced 4-CBA subsequently co-crystallizes with TA, a TA product that contains undesirable impurities is obtained.
Moreover, Pd/C catalyst beds become less active as they are used. Eventually such beds must be taken out of service as product specifications for purified TA are exceeded.
Accordingly, it would be desirable to reduce the overall 4-CBA level in purified TA and to extend the useful life of a Pd/C catalyst bed for producing purified TA having a specified low level of 4-CBA derived impurities. The present invention satisfies both of the foregoing desiderata.